1. Technical Field of the Invention
The present invention relates to a light-weight ceramic acoustic absorber used for the exhaust nozzles etc. of a jet engine, which is light in weight and has excellent resistance to thermal stresses and excellent sound absorbing properties, and also to a method of manufacturing the same.
2. Description of the Prior Art
An acoustic absorber, with the honeycomb structure shown in FIG. 1 as an example, has been used conventionally as an acoustic absorber for the exhaust nozzles etc. of a jet engine. This honeycomb acoustic absorber consists of a honeycomb 1 of a heat-resistant alloy, a perforated plate 2 and a back plate 3, and this is a reactive type of Helmholtz resonator; the absorber dissipates energy through losses caused by the friction of a medium with a wall surface and the relative motion of the medium to the surface.
However, this type of honeycomb acoustic absorber suffered from problems such as overheating of the perforated plate 2 or the internal honeycomb 1 or the back plate 3 by exhaust gas at a high temperature (for example, in the range of 700 to 800 K or greater), or excessive thermal distortion. More specifically, the perforated plate 2 often consists of, for example, a stainless steel or aluminum plate, therefore the plate may be damaged or thermally deformed when overheated, and furthermore, the portion where it is welded to the honeycomb 1 might become detached. More seriously, with this sound absorption structure, the bandwidth of the absorbable noise is so narrow that noise with a large bandwidth from a jet engine etc. (for example, in the range of 1000 to 3,000 Hz) cannot be absorbed completely.
On the other hand, a resistance type acoustic absorber consisting of a perforated layer and a fiber layer, is also known in the prior art, and various related embodiments have been proposed (for example, Japanese Unexamined Patent Publications Nos. 61-143501, 61-44102, 6-42071 and 6-247778).
However, the above-mentioned Japanese Unexamined Patent Publication No. 61-143501 by the title of "Manufacturing method of perforated acoustic absorber" and the Japanese Unexamined Patent publication No. 61-44102 by the title of "Light-weight, high-strength acoustic absorber" relate to a material composed of metal particles, together with a residual void ratio as low as 20.about.50%, therefore they have problems which include a low heat resistance and large specific and total weights.
The Japanese Unexamined Patent Publication No. 6-42071 by the title of "Ceramic acoustic absorber" relates to a material composed of ceramic, so its resistance to thermal stresses is poor despite its high resistance to heat, because the structure as a whole, is isotropic, and the material cracks easily. Moreover, the sound absorption ratio of the material is only 60% or less in the frequency range of 1,000 to 2,000 Hz, which is a particular problem with regard to the noise from a jet engine. Consequently, this material was not satisfactory.
In accordance with the Japanese Unexamined Patent Publication No. 6-247778 by the title of "Light-weight ceramic molding with voids with controlled diameters, and a method of manufacturing the same", the growth of cracks can be delayed by virtue of the graduated location of the voids, but the sound absorption performance is not satisfactory. Also when the invention is applied to a jet engine, the molding can easily be broken off if the surface of the molding is overheated by a gas jet.